The invention relates to a method of elongating a tube by way of a drawing ring, and to a device for elongating a tube. Elongating methods of said type are used in those cases where there is a need for one-piece (non-assembled) tubular members whose wall thicknesses differ in the longitudinal direction. Such applications are so-called mono-block intermediate shafts, for example, which comprise a longer central portion with a smaller wall thickness and two end portions with a greater wall thickness and, at the same time, with a smaller outer diameter. For producing such shafts it is common practice to start with a straight cylindrical blank, i.e. a simple tubular portion of which, first, the front end is reduced in order to form the first tube end, then to elongate the major part of the tube length over a mandrel in order to produce the central portion with a smaller wall thickness and finally to reduce the rear, non-elongated tube end in order to produce the second tube end. An example of this is shown in German patent DE 35 06 220 A1.
Elongation takes place in such a way that a mandrel is inserted into the tube from the rear end of same. The mandrel supports the tubular member from the inside, and the front end of the mandrel, on the inside, rests against the reduced front end portion of the tubular member.
The mandrel together with the surrounding tubular member is then pushed through a drawing ring. While the inner diameter of the tubular member is supported, the outer diameter is reduced, with the tube simultaneously being elongated when passing through the effective cross-section of the drawing ring. From the beginning of the process, the reduced tube portion is subjected to tensile forces, with the tensile forces being higher the greater the degree of deformation, i.e. the greater the change in diameter or the greater the speed of feed respectively.
Thus, there exists a need for an improved tube drawing process to increase tool service life and/or shorten drawing cycle times.